Present day data communication networks, both wireless and wire-line, have a requirement to transfer data between communication units. Data, in this context, includes many forms of communication, such as speech, multimedia, signalling, etc. Typically, such data communication needs to be effectively and efficiently transported, in order to optimise use of limited communication resources.
Due to the recent growth in communications, particularly in Internet and wireless communications, there exists a need to provide improved data transfer techniques, where a particular quality of service of the transmitted data is often required or desired by the end user.
The European Telecommunication Standards Institute (ETSI) has defined a number of communication standards with the aim that a number of manufacturers are able to provide equipment that supports the same technology and notably are able to inter-operate with other equipment compliant with that standard. One such data communication standard developed by ETSI is the Terrestrial Digital Video Broadcasting (DVB-T) standard (ETSI EN 300 744), which has been developed for digital television sets and set-top boxes.
A recent variation of the DVB-T standard that has been adopted to incorporate enhanced features to allow improved reception of digital video broadcasting services for mobile devices is the digital video broadcasting—handset DVB-H standard. A DVB-H unit is battery powered, and the nature of the broadcast transmission offers a possibility to the DVB-H unit to repeatedly power off components/circuits of the DVB-H unit's receiver chain to increase battery life. It is anticipated that DVB-H units may receive transmissions at a variety of locations, such as: indoor, outdoor, as a pedestrian, within a moving vehicle, etc.
Historically, DVB-T was targeted for MPEG-2 video to be transmitted in MPEG-2 Transport Steams (TS), with the MPEG-2 TS protected with Reed Solomon (RS) Forward Error Correction (FEC) codes. To cope with mobile propagation degradation, DVB-H introduced another layer of RS FEC called multi-protocol encapsulation (MPE)-FEC. Here, only MPE blocks with correct cyclic redundancy check (CRC) are further processed by the MPE FEC decoder. If the CRC fails, the whole block is discarded. Zeros are then inserted at the proper byte positions in the RS code words, instead of the block data, and are marked as “unreliable”. If there are more than 64-unreliable byte positions in an RS code word, the RS decoder cannot correct anything, and therefore just outputs the bytes without error correction.
Referring now to FIG. 1, there is illustrated an example of part of a known decoder 100 for decoding received DVB-T (or similar) signals. The decoder 100 comprises an inner decoder 110 and an outer decoder 120 for decoding the received demodulated signal. As is known in the art, the inner decoder is a convolutional decoder, in which the reception is typically implemented using a soft-decision Viterbi decoder. This reduces the effect of thermal noise and interference on the quality of the received signal, as Viterbi errors are generally bursty in nature. A Reed-Solomon decoder is used as the outer decoder, which feeds the decoded signal into a descrambler 130, which descrambles the received data packets, which are then output, for the illustrated example, in the form of MPEG-2 transport multiplex (MUX) packets.
Transmitted data is scrambled in order to ensure adequate binary transitions for the purpose of energy dispersal. A pseudo random binary sequence (PRBS) is used to scramble MPEG-2 transport multiplex packets. A synchronisation (sync) byte is then added to the front of each scrambled packet. The sync byte of the first packet in each group of eight packets is bit wise inverted to provide an initialisation signal for the descrambler. In this manner, each time a packet comprising an inverted sync byte is received, typically every 8th packet in a stream, the descrambler reinitialises the PRBS.
Accordingly, for the example illustrated in FIG. 1, after decoding the received data, the outer decoder 120 passes decoded sync bytes to synchronisation logic 140, which detects inverted sync bytes, and signals the descrambler when to reinitialise the PRBS, for example by way of a modulo 8 index signal.
The MPEG-2 transport MUX packets are then passed to a de-multiplexer (not shown), which de-multiplexes each MPEG-2 transport MUX packet, to separate out the individual services encapsulated therein. A problem with MPEG-2 transport MUX packets is that information required to de-multiplex each packet is located within the header section of that packet. As a result, if errors occur within the header section during transportation of the MPEG-2 transport MUX packet from its source to its destination, it may not be possible to de-multiplex the packet. Consequently, the packet will be discarded, losing all the data carried therein, even if the data itself is intact and/or useful.
As will be appreciated by a skilled artisan, although it is possible to add redundancy and additional forms of forward error correction and the like to the transmission of data signals, the addition of such redundancy etc. means that such transmissions no longer conform to existing standards, and thus introduces compatibility issues.